Of course they do, and politicians take bribes. That doesn't make it right.
Are you saying that offering immunity is a practice that needs to outlawed? You might as well throw out every prosecution of any organized crime figure. you think they are hanging around with the nuns?
Of course bribing a witness should be outlawed. The same with bribing politicians. Just because the government does it doesn't make it right. Do you need to be bribed to testify if you witness a crime?
So you think the government can't make its case without bribing its witnesses? Wouldn't it be a lot easier too if the government just assumed guilt and made the criminal testify? That damned fifth amendment makes it real hard to convict criminals too. (Sarcasm)
Maybe the government needs to reexamine the laws it is trying to enforce. It is the laws against what consenting adults do that is perverting the justice system.